The interest in crystalline cadmium sulfide for use in semiconductor and optical devices has engendered a corresponding interest in producing defect-free cadmium sulfide crystal surfaces. For example, the sensitivity of the CdS phonon spectrometer developed by Shah et al., (Phys. Rev. Letters, 33 (14), 818 (1974)), depends strongly on the surface quality of both the A and B face of the CdS crystal employed. Similarly, the optical qualities of a ZnO layer deposited on either the A or B face also depend on the quality of the CdS deposition surface. (See Vac. and Polito, J. Va. Sci. and Tech, 6 (1), 115 (1969)).
Various methods have been investigated for CdS polishing. For example, Warekois et al., (J. Appl. Phys., 33 (2), 690 (1961)) used a chemical etchant of 0.5M K.sub.2 Cr.sub.2 O.sub.7 in 16N sulfuric acid as a polishing agent. However, dislocation etch pits on the sulfur surface (B) and shallow dishes on the cadmium (A) surface were reported. Other researchers have used a combination of mechanical and chemical steps to obtain a smooth surface. A method described by Sullivan and Bracht (J. Electrochem. Soc. 114 (3), 295 (1967)) entails first lapping the crystal with 5.mu.m grit Al.sub.2 O.sub.3 and then polishing with a Pellon cloth in the presence of hydrochloric acid (an aqueous solution of between 10 and 50% of 37% HCl in water). This procedure yielded a surface flat to within 0.1 mils.
Multicomponent etchants have also been used in a process involving both mechanical and chemical polishing. Pickhardt and Smith (J. Electrochem. Soc., 121 (8), 1064 (1974)) have reported a process involving first mchanically polishing with a 3.mu.m abrasive followed by polishing with a multicomponent mixture capable of both chemical and abrasive polishing. The multicomponent mixture contained 90 ml HNO.sub.3, 300 ml precipitated silica, 10g AlCl.sub.3 and 1 ml concentrated detergent solution per 1000 ml of deionized water. This process yielded A faces which were flat to 300 Angstroms. However, B faces polished with the same composition had scratches and an orange-peel texture. Slightly better B face surfaces were obtained by using a polishing agent consisting of 150 ml HCl and 5g AlCl.sub.3 per 1000 ml of water. The Pickhardt and Smith polishing process even on the A face can yield significantly degraded results with changes in the complex nature of the agent. Increases or decreases in acid concentration, increases in the silica content, or deletion of AlCl.sub.3 all produce surface irregularities. The composition of the polishing agent must, therefore, be very carefully controlled. This is an undesirable aspect of the method.
Despite these varied efforts a process involving a simple polishing agent capable of producing good surfaces, i.e., flat to less than 300 Angstroms on both the A and B faces in cadmium sulfide, for such applications as electrooptical devices, has not been developed. The continued investigation and use of devices whose quality depends on the surface properties of the CdS crystal utilized, emphasizes the need for a simple, easily controllable process for producing highly polished surfaces.